Golf balls traditionally have been categorized in three different groups, namely as one-piece, two-piece and three-piece balls. Conventional two-piece golf balls include a solid resilient core having a cover of a different type of material molded thereon. Three-piece golf balls traditionally have included a liquid or solid center, elastomeric winding around the center, and a molded cover. Solid cores of both two and three-piece balls often are made of polybutadiene and the molded covers generally are made of natural balata, synthetic balata, or ionomeric resins.
Ionomeric resins are polymers containing interchain ionic bonding. As a result of their toughness, durability and flight characteristics, various ionomeric resins sold by E.I. DuPont de Nemours & Company under the trademark "Surlyn.RTM." and by the Exxon Corporation (see U.S. Pat. No. 4,911,451) under the trademark "Escor.RTM." and the trade name "lotek", have become the materials of choice for the construction of golf ball covers over the traditional "balata" (transpolyisoprene, natural or synthetic) rubbers. The softer balata covers, although exhibiting enhanced playability properties, lack the durability (cut and abrasion resistance, fatigue endurance, etc.) properties required for repetitive play.
Ionomeric resins are generally neutralized copolymers of an olefin, such as ethylene, and an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid or maleic acid. Metal ions, such as sodium or zinc, are used to neutralize some portion of the acidic group in the copolymer, resulting in a thermoplastic elastomer exhibiting enhanced properties, i.e., durability, etc., for golf ball cover construction over balata.
While there are currently more than fifty (50) commercial grades of ionomers available from Exxon and DuPont, with a wide range of properties which vary according to the type and amount of metal cations, molecular weight, composition of the base resin (i.e., relative content of ethylene and methacrylic and/or acrylic acid groups) and additive ingredients such as reinforcement agents, etc., a great deal of research continues in order to develop a golf ball cover composition exhibiting the desired combination of playability properties.
Golf balls are typically described in terms of their size, weight, composition, dimple pattern, compression, hardness, durability, spin rate, and coefficient of restitution (COR). One way to measure the COR of a golf ball is to propel the ball at a given speed against a hard massive surface, and to measure its incoming and outgoing velocity. The COR is the ratio of the outgoing velocity to the incoming velocity and is expressed as a decimal between zero and one.
There is no United States Golf Association limit on the COR of a golf ball but the initial velocity of the golf ball must not exceed 255 ft/second. As a result, the industry goal for initial velocity is 255 ft/second, and the industry strives to maximize the COR without violating this limit. Having the longest ball--compatible with the U.S.G.A. requirements--has been and also remains another, longstanding objective of golf ball manufacturers. In this respect, prior balata and polymeric covered balls, and certainly those intended for U.S.G.A. regulation play, have shared one thing in common. They have all relied on their preformed cores as the primary vehicle for transferring energy from the golf club to the ball when the ball is struck by the club. For years, the principal thrust of golf ball research and development has been directed to making improved preformed cores for enhancing distance performance. In other words, conventional wisdom among golf ball manufacturers has been that enhanced distance performance is primarily achievable through the use of better energy transferring cores.
This is not to say that some did not recognize that the cover composition could contribute to the ball's distance performance. U.S. Pat. No. 3,819,768, which issued in the name of Robert P. Molitor (one of the named inventors herein) and which disclosed the use of mixed SURLYN.RTM. resins as a ball cover material, recognized that the use of a mixed SURLYN.RTM. cover could and did increase the distance the ball could be driven. Nevertheless, the cores of the balls manufactured under U.S. Pat. No. 3,819,789 still were preformed and were believed to be a significant contributor to energy transfer. The coefficient of restitution of these cores was approximately 0.750 and the final ball was approximately 0.780.
The approach of utilizing preformed cores as the principal "mechanism" for transferring energy has recognized disadvantages. The cost of manufacturing such preformed cores is a relatively large part of the cost of manufacturing the golf balls. Because of the relative complex technology and operations involved, the use of such preformed cores have also imposed added quality assurance problems and costs.
In summary, those working in the golf ball art have long sought to develop a golf ball that: has the "feel" and controllability of a balata covered three-piece ball; has a high initial velocity or initial coefficient of restitution, has a good "cut resistance" and durable cover; may be driven long distances in regulation play, hopefully by "average" golfers; and, importantly, may be uniformly and inexpensively manufactured by mass production techniques. As noted, to a large extent, the industry has sought to achieve this long recognized and desired objective by using a polymer or balata cover over a preformed core and by enhancing the ball's preformed core's capacity for transferring energy when the ball is struck by a golf club.
A number of patents have issued which disclose golf balls having multi-layer cores or covers. U.S. Pat. Nos. 4,431,193 and 4,919,434 disclose golf balls with multi-layer covers. U.S. Pat. No. 4,431,193 discloses a multi-layer ball with a hard inner cover layer and a soft outer cover layer. U.S. Pat. No. 4,919,434 discloses a golf ball with a 0.4-2.2 mm thick cover made from two thermoplastic cover layers. U.S. Pat. No. 5,273,286 discloses a golf ball with a multi-layer core. The golf ball disclosed therein has an inner core, a shell surrounding the inner core, an outer core, and a cover.